Highcharts iOS wrapper provides its own colors implementation. As you can notice, some options are of HIColor type. You can instantiate the desired color in a few ways which are described in the API documentation

Here, we will show the most complex case which is gradient usage. For example, you can instantiate a color for chart background:

HIChart *chart = [[HIChart alloc]init];
chart.backgroundColor = [[HIColor alloc]initWithLinearGradient:@{ @"x1": @0, @"x2": @0, @"y1": @0, @"y2": @300 } stops:@[
    @[@0, @"rgb(102, 153, 161)"],
    @[@1, @"rgb(128, 135, 232)"]

Original info at the Highcharts iOS GitHub repository